Metal plating of thermoplastics

ABSTRACT

A method of electrolessly plating a thermoplastic substrate with a metal, such as copper, nickel or cobalt, prior to application of an electrolytically deposited metal. A novel surface treatment step which includes an improved neutralizer for the conventional chrome-containing chemical etch, is effective in promoting electroless metal coverage and the adhesion between the electroless metal layer and the substrate.

' uited States Patent Inventors Randy L. Bauer Marietta, Ohio; David C. Johnson, Washington, W. Va.; Ronald M. Noss, Parltersburg, W. Va. App]. No. 793,193 Filed Jan. 22, 1969 Patented Nov. 16, 1971 Assignee Borg-Warner Corporation Chicago, Ill.

METAL PLATING 0F Tl-IERMOPLASTlCS 7 Claims, No Drawings Primary Examiner-Alfred L. Leavitt Assistant Examiner-Janyce A. Bell AttorneysDonald W. Banner, Lyle S. Motley, C. G. Stallings and William S. McCurry ABSTRACT: A method of electrolessly plating a thermoplastic substrate with a metal, such as copper, nickel or cobalt, prior to application of an electrolytically deposited metal. A novel surface treatment step which includes an improved neutralizer for the conventional chrome-containing chemical etch, is effective in promoting electroless metal coverage and the adhesion between the electroless metal layer and the substrate.

METAL PLATING OF THERMOPLASTICS BACKGROUND AND SUMMARY OF THE INVENTION This invention relates generally to the electroless deposition of metal onto a chemically etchable, nonconductive substrate as a step preceding the electrolytic deposition of metal thereon. More particularly, the invention relates to a method of increasing the coverage and the adhesion between the electroless metal layer and the substrate, which is particularly useful in the processing of difficult to plate substrates, such as aliphatic polyolefins.

In the present invention, it has been found that the use of aqueous ammonia, amines, and other solutions capable of complexing trivalent chromium, as chrome etch neutralizers, can substantially increase the metal to substrate coverage and adhesion.

The basic 3) common to practically all known plastic plating processes may be generally stated as follows: (1) preparation of surface by chemical etching or mechanical roughening, (2) catalytic sensitization with noble metal salts, e.g., PdCl (3) electroless metal deposition, e.g., using a nickel-sodium hypophosphite bath, and (4) electrolytic deposition of metal. The principal factor affecting the electroless metal coverage and adhesion in this type of process is step (1) above; for, if the surface is not properly prepared, misplate may occur.

in commercial processes for plating of plastics, the substrate is normally cleaned in a dilute alkaline solution to remove grease and surface grime picked up during handling of the parts prior to plating. The next step involves the use of a strong oxidizing acid etch containing, for example, sulfuric and/or phosphoric acid and chromic acid. The chemical etchs which have been found to be most effective in treating a wide variety of thermoplastic substrates all contain chromic acid. While the surface phenomena are not entirely understood, it is believed that the oxidizing effect of the chemical etch forms epoxy or carboxylic acid groups on some of the carbon to carbon linkages, which are more readily activated by the catalyst used as the next step of the process.

The present invention concerns itself with the recognition that the use of chromic acid in the etch has an undesirable side efl'ect. it is believed that some trivalent chromium in the etch coordinates with the epoxy oxygen or the carboxylic group in various ways. The reaction cannot be easily identified, but it is theorized that the ion formed may contain 2 to 3 water ligands, or may be further combined with the sulfate anion in the case of a sulfuric-chromic acid etchant.

The trivalent chromium, in coordinating with the oxygen, apparently prevents the catalyst from activating the surface in a uniform manner. In the prior art, the step following the chemical etch is usually a mild acid bath. It is believed that the complex reaction, whereby the trivalent chromium coordinates with additional functional groups, makes it very difficult for the acid type "neutralizer (for neutralizing" chromium compounds) to have much effect at all on certain plastic substrates, such as polypropylene. Accordingly, the thrust of the present invention is directed to the use of a complexing agent which has the ability to remove trivalent chromium from the surface. In addition to known chelating agents, such as ethylenediamine, aqueous ammonia and other amine derivatives may also be effective.

Accordingly, it is a principal object of the invention to provide an improved process for the electroless deposition of metal, such as, for example, nickel or cobalt, onto a thermoplastic substrate.

Another object of the invention is to provide a novel chrome neutralizer for the chrome-containing chemical etch commonly employed in the surface treatment phase of the plating process.

Another object of the invention is to reduce the time required for the etch cycle, which, in the case of hard to plate substrates, is presently minutes or more.

Additional objects and advantages will be apparent from reading the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION For purposes of this disclosure, reference will be made to a plateable thermoplastic substrate. In the present state of the art, the most successful of such substrates are acrylonitrile-butadienee-styrene (ABS) graft copolymers, polysulfones, and polypropylene. Other thermoplastics that also lend themselves to metal plating by the process of this invention are other polyolefins including polyethylene,

poly(4-methyl-l-pentene), poly-( l-butene), and copolymers such as ethylene-propylene copolymer; polystyrene; poly(phenylene oxide); polycarbonates;

copolymers of acrylonitrile and styrene (polySAN); ABS-like plastics including acrylate esterbutadiene-styrene graft polymers and interpolymers, blends of nitrile rubber and polySAN, and ABS-type interpolymers and graft polymers in which one or more of the monomers is substituted with halogen or hydrocarbyl groups. it is not particularly important as to what substrate is used as the function of the chrome neutralizer is essentially the same in any case, Virtually all commercial plastics plating processes utilize a chemical etch which has, as one of its ingredients, chromic acid. It is the problem of removing trivalent chromium adsorbed onto the surface to which the present invention proposes a solution.

The process begins with general cleaning of the articles to remove surface dirt, grease, etc., usually in a mild alkaline bath. This step does not have any direct effect on metal adhesion. After a water rinse, the parts are then immersed into the chemical etch which contains a mixture of sulfuric acid, chromic acid, and sometimes phosphoric acid. The part is then washed in water and immersed in the chrome neutralizer bath.

After being washed again, the electroless plating phase begins with a catalyst bath followed by an accelerator. The electroless deposition process may be of several known types, such as, for example, those described in the specification of U.S. Pat. No. 3,01 1,920 issued to C. R. Shipley, Jr. on Dec. 5, 1961. In the conventional electroless plating process, metal deposition takes place in two stages: (1) sensitization of the substrate surface by means of a catalytic metal; and (2) deposition of the primary metal by means of a reducing agent.

in general, the surface of the substrate, after being properly etched by the process described above, is immersed into a bath containing the catalytic metal; and thereafter, the catalyzed substrate is removed to a bath containing the deposition solution which ordinarily contains a salt of nickel, cobalt, copper, silver, gold, chromium or members of the platinum family, and a reducing agent, commonly formaldehyde, a hypophosphite salt, or dimethylamineborane.

The various combinations of catalyst and deposition metals are known in the art. For example, the following have been reported to be catalytic to the deposition of nickel and cobalt: copper, beryllium, aluminum, carbon, tungsten, tellurium, cobalt, platinum, silver, boron, thallium, vanadium, titanium, nickel, gold, gennanium, silicon, molybdenum, selenium, iron, tin and palladium, with the precious metals gold, palladium and platinum being preferred. The same catalysts promote the deposition of copper, lead, platinum, rhodium, ruthenium, osmium, iridium, iron, silver, aluminum, gold, palladium, and magnesium, with gold, platinum and palladium being preferred. Cobalt, nickel, and particularly iron have also been used to catalyze the deposition of chromium.

After removal from the electroless plating bath, the article is rinsed to remove residual electroless metal solution, and then electrolytically plated by any conventional method. One preferred series of electrolytically applied metals is l copper (to reduce the effect of thermal shock), (2) nickel, and (3) chromium. It should be understood, however, that any electrolytically deposited metal, or combination thereof, may be used in conjunction with the process.

As mentioned earlier, the present invention is directed primarily to the use of an improved chrome neutralizer which is capable of forming a complex with trivalent chromium. Examples of such compounds would include ammonium hydroxide, ethylenediamine and other polyamines, EDTA and other carboxy-amino poly acids, amino acids such as glycine, hydroxyamines such as mono-, di-, and triethanolamine; hydroxycarboxylic acids, such as glycolic, citric, and tartaric acids; and possibly chelating phosphates such as pyrophosphon'c acid.

To further illustrate the invention, reference is made to the following examples which are intended to be illustrative, and not in any sense limiting. Where adhesion values are stated, it should be pointed out that all such values are measured in accordance with the description contained in the specification of application Ser. No. 494,861 filed in the name of K. A. Klinger et al. on Oct. 11, 1965 and now Pat. No. 3,445,350.

The examples described below involve the nickel plating of plastics using the procedures outlined above. The examples differ only in some features which are part of the preparation for plating steps 1 through 6.

EXAMPLE I Articles molded from plating grade ABS (CYCOLAC EP-3510-subjected to the following preplating steps:

1. Immersion for 5 minutes in a mild alkaline cleaner bath at 2. Rinse in a cold water bath.

3. lmmersion for 5-20 minutes at 135-145 F. in a chemical etching bath containing:

47 wt% H 25 wt% H 80. (93%) 4. Rinse in a cold water bath.

5. Immersion for 14-2 minutes at room temperature in an acid type chrome neutralizer bath.

6. Rinse in a cold water bath.

The following electroless and electrolytic plating steps, which were used in all of the examples, are representative of several conventional plastic plating systems which are commercially available.

7. Catalyst-as described in U.S. Pat. No. 3,01 1,920, at

room temperature for 1 to 8 minutes.

8. Cold water rinse.

9. Acceleratoras described in US. Pat. No. 3,01 1,920, at

65-140 F. for 1 to minutes.

10. Cold water rinse.

l l. Electroless nickel-an alkaline nickel bath containing hypophosphite salts and conventional complexing and stabilizing compounds (Marbon N-35) at room temperature for 3 to 8 minutes.

12. Cold water rinse.

13. Conventional electroplating.

Additional parts were processed varying the composition of the bath of step 3 from 25-50 wt% H O; from 5-30 wt% CrO and from 20-70 wt% H 80 (93%).

After completion of the plating process, the plated parts had adhesion values of 1 l-l 3 pounds per inch strip and complete coverage.

EXAMPLE 11 The same type of molded ABS articles as those used in example was used in this example. Steps 1, 2, 3, and 4 were the same as in example 1. The chrome neutralizer of step 5 was composed as follows:

vol% NH OH solution (26 Be) 90 vol% H O The immersion time for step 5 was 2 minutes. After step 6 (same as in example 1), the parts were plated according to steps 7 to 12. Complete coverage of parts and adhesion values of 14-15 pounds per inch strip were achieved.

EXAMPLE lll Molded articles of another plating grade ABS Marbon EPA-3530) were processed following the routine described of example 1. Eighty percent coverage was attained. These parts could not be adhesion tested.

EXAMPLE IV Molded articles described in example 111 were processed following the procedure described in example 11. Complete coverage was achieved.

EXAMPLE V Molded articles of plating grade polypropylene are prepared for plating by using the following sequence of steps:

1. Same as example I.

2. Same as example 1.

3A. Same as step 3 of example 1.

3B. Immersion for 20-25 minutes at 160 F. in a chemical etching bath containing:

17 wt% H 0 1 wt% CrO 52 wt% H 50 (93%) 4. Same as example 1.

5. Same as example 1.

6. Same as example 1.

Additional parts were processed varying the composition of the bath of step 38 from 5-25 wt% H O; from 1-30 wt% CrO from 20-70 wt% H (93%); and from 10-50 wt% H OP (84%) immersion time ranged from 20-25 minutes. The plated parts had only 50-60% coverage.

EXAMPLE V1 The procedure of example V was followed in plating more of the same polypropylene articles except that step 5 operated at 35 F. and 70 F. and used a chrome neutralizer composed as follows:

l0 vol% Nl-LOH solution (26 Be) vol% H O After plating, specimens had at least 99% coverage and adhesion values of 26.6 pounds per inch strip.

EXAMPLE Vll The procedure of example VI was followed except that the chrome neutralizer bath of step 5 was varied in composition between 5 vol% and 50 vol% Nl-LOH. Over 99% coverage was achieved in all cases.

EXAMPLE Vlll Articles of plating grade polypropylene were processed in the manner described in example Vl excepting the normal etch time of 20-25 minutes (step 2) was reduced to 5, l0 and 15 minutes. Respective adhesion values of 16.0, 24.0, and 29.3 pounds per inch strip were obtained and all samples showed 99-100% coverage.

EXAMPLE 1X A solution of 134 g. of ammonium acetate per liter of water was used as a neutralizer. The bath was maintained at about F. The treatment times ranged from 1 to 3 minutes. Electroless nickel coverage ranged from 85-90%.

EXAMPLE X Ammonium nitrate in concentrations of 134 g. per liter, 268 g. per liter, and 402 g. per liter were used on three polypropylene samples. Treatment times ranged from 1 to 3 minutes, and electroless nickel coverage was obtained on the average of about 85%.

EXAMPLE Xl Aqueous nickel ammonium sulfate was employed as the neutralizer in substantially the same manner as example Vl, i.e., 134 g. per liter. Coverage in excess of 85% was obtained.

EXAMPLE xn Aqueous sodium ammonium phosphate was employed as neutralizer in substantially the same manner as example VI, i.e., 134 g. per liter. Coverage in excess of 99% was obtained.

EXAMPLE XIII EXAMPLE XIV Ammonium phosphate (monobasic) was employed in substantially the same manner as example VI using 350 g. per liter. Coverage of substantially 100% was obtained.

EXAMPLE XV Example VI was repeated except that ethylenediamine in aqueous solution was substituted as the chrome neutralizer. Samples were run from immersion times of l, 2, and 3 minutes and coverage was in excess of 99%. Adhesion values for samples subsequently plated were on the order of -12 lbs/inch.

EXAMPLE XVI Example VI was repeated except that triethanolamine in aqueous solution was substituted as the chrome neutralizer. Samples were run from immersion times of l, 2, and 3 minutes, and coverage was in about 70-85%.

EXAMPLE XVII Example VI was repeated except that hexamethylenetetramine in aqueous solution was substituted as the chrome neutralizer. Samples were run from immersion times of 1, 2, and 3 minutes, and coverage was in excess of 99%.

EXAMPLE XVIII Example VI was repeated except that mixtures of hexamethylenetetramine and ethylenediamine in aqueous solution were used as the chrome neutralizer in concentrations ranging from 10% ethylenediamine, to hexamethylenetetramine, up to 50% of each component. All samples were diluted with enough water to form 5% solutions. Electroless nickel coverage of all samples was approximately 100%.

EXAMPLE XIX Cirtic acid was tested in a process similar to example XVIII. Electroless nickel coverage was approximately 100%.

EXAMPLE XX A 5% aqueous solution of disodium salt of ethylenediaminetetraacetic acid was tested in a process similar to example XVIII. Electroless nickel coverage was in excess of While the invention has been described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not by way of limitation; and the scope of the appended claims should be construed as broadly as the prior art will permit.

What is claimed is:

1. A method of preparing the surface of a thermo plastic substrate prior to electroless deposition of metal thereon comprising the steps of treating said surface with a chemical etchant containing chromic acid, and subsequently complexing any trivalent chromium adhering to the surface by treating said surface with an aqueous composition selected from the group consisting of ammonium acetate, ammonium nitrate, nickel ammonium sulfate, sodium ammonium phosphate, am-

monium phosphate (diabasic ammonium phosphate (monobasic and citric acid.

2. A method as defined in claim 1, wherein said composition contains ammonium acetate.

3. A method as defined in claim 1, wherein said composition contains ammonium nitrate.

4. A method as defined in claim 1, wherein said composition contains nickel ammonium sulfate.

5. A method as defined in claim 1, wherein said composition contains sodium ammonium phosphate.

6. A method as defined in claim 1, wherein said composition contains ammonium phosphate (diabasic).

7. A method as defined in claim 1, wherein said composition contains ammonium phosphate (monobasic).

* i i i 

2. A method as defined in claim 1, wherein said composition contains aMmonium acetate.
 3. A method as defined in claim 1, wherein said composition contains ammonium nitrate.
 4. A method as defined in claim 1, wherein said composition contains nickel ammonium sulfate.
 5. A method as defined in claim 1, wherein said composition contains sodium ammonium phosphate.
 6. A method as defined in claim 1, wherein said composition contains ammonium phosphate (diabasic).
 7. A method as defined in claim 1, wherein said composition contains ammonium phosphate (monobasic). 